I've used it for several years and the feedback from a user point of view is not positive. My sample size includes me and several members of my family who used Sony Xperia devices running SailfishOS for several years.
The Sailfish guys for some odd reason decide to invent their own "user interactions" where you click-slide ("one handed") to do certain opertaions. This makes the UI not only awkward, but NOT intuitive. You don't know what your options are until you perform this strange operation. I get why they did this, it was a way to potentially reduce swiping, etc but now that we have phones with big screens, you can actually put those options in one UI.
Further, basic things like composing a text and attaching a photo requires a round-trip to the photo app where you 'tag' the images you want ONE BY ONE rather than being able to do this inline from the SMS/MMS application. I think this has gotten better recently but for a long time it was SUPER awkward.
Two other perplexing points was how SLOW the UI felt for what should have been compiled Qt code and poor battery life on the older Xperia devices. Maybe they're using QML and it's not compiled?
The Sailfish guys have what I think is an ugly looking UI as well.
They've "dithered" certain parts of the UI so it really looks like old-school EGA/CGA graphics, even though the display is high-DPI and they have what's effectively a TUI style interface.
The only people I know who "LOVE" or claim "it's the best" UI are the same ones who LOVE Zune and Windows Phone UIs which are basically flat UI, almost monocolor nearly TUI type which is what you see pieces of in Win10 as well. Personally I dislike this UI and so do many people I know, there's a reason why UIs have icons and ideally text labels. TUIs have their place but so do GUIs.
If the Sailfish guys abandoned their weird UI ideas and frankly made it more like iOS or Android (I know, so boring, we have to re-invent the wheel just because...) it would actually be compelling.
On the very very plus side of Sailfish, as someone else pointed out, it's basically a GNU/Linux device that uses RPMs. I was able to install dnsmasq, set up DNS based adblock filtering, curate firewall rules and basically harden the device. You could SSH into the device via USB without adb stupidity and once I set it up, it stayed working until the VOLTE switch-over occured.
I think Ubuntu Touch has a better "UI" (I've also run this) but the Ubuntu guys have basically been ignoring VOLTE and since all major US carriers have switched over to VOLTE, your phone basically can't really make calls now on Ubuntu Touch (but that's OK, they've improved a bunch of other stuff! /sarcasm off).
Ubuntu Touch (not that you asked) is also a LOT slower than it should be and because the Ubuntu Touch guys are pursuing an 'Over the Air' update model, since the OS can basically be overwritten, applications aren't actually unpacked at install time but dynamically at run time. On a desktop this is OK but on a phone it leads to very slow app loading times.
I have high hopes for the current batch of Linux phone projects, Mobian, postmarketOS, etc but sadly I'm on Android until these are fully solidified.